1. Technical Field
The present invention relates to a common mode filter, and more particularly, to a common mode filter, in which electro static discharge (ESD) protection function is provided.
2. Description of the Related Art
With the development in technologies, electronic apparatuses such as mobile phones, home appliances, personal computers (PC), personal digital assistants (PDA), liquid crystal displays (LCD), and the like have changed from an analog scheme to a digital scheme, and have been speeded up due to an increase in the amount of data to be handled. Therefore, USB 2.0, USB 3.0, and a high-definition multimedia interface (HDMI) have been commonly used as a high signal transmission interface, and have been used in many digital devices such as PCs or digital high-definition televisions.
These interfaces adopt a differential signal system for transmitting differential signals (differential mode signals) using a pair of signal lines unlike a single-end transmission system that has been typically used for a long time. However, electronic apparatuses which have been digitalized and speeded up are sensitive to external stimulation. That is, when a slightly abnormal voltage and high frequency noise flow into an internal circuit of the electronic apparatus from the outside, the circuit may be damaged or signals may be distorted.
In this instance, as the causes of the abnormal voltage and noise that cause damage of the circuit of the electronic apparatus and occurrence of signal distortion, lighting, discharge of static electricity charged on a human body, switching voltage generated within the circuit, power supply noise included in a power supply voltage, unnecessary electromagnetic signals or noise, and the like may be given. In this manner, in order to prevent the damage of the circuit of the electronic apparatus and occurrence of the signal distortion, a filter is provided to prevent the abnormal voltage and high-frequency noise from flowing in the circuit.
In general, a common mode filter is used in a high-speed differential signal line or the like in order to eliminate common mode noise. The common mode noise is noise generated in the differential signal line, and the common mode filter eliminates noises that cannot be eliminated by an existing EMI filter. The common mode filter contributes to the improvement in EMC characteristics of home appliances or the like or improvement in antenna characteristics of a mobile phone or the like.
However, in the recent high-speed digital interface, since the interface handles minute signals transmitted at a high speed, an integrated circuit (IC) that is significantly sensitive to electro static discharge (hereinafter, referred to as “ESD”) generated at the time of connection and disconnection between different elements should be used.
In order to prevent damage of the IC by ESD, a varistor may be used as a device for coping with the ESD between a signal line and a base. However, when the varistor is used, a signal waveform is deactivated, and signal quality is deteriorated. Therefore, a lower capacity-device for coping with ESD is required.
For example, in Korean Patent Laid-Open Publication No. 10-2010-0037000, a structure in which a lead wire connected with an external electrode is spaced apart from a ground electrode for eliminating ESD (hereinafter, referred to as “ground electrode”) by a significantly small interval is used. This interval is generally constituted of a dielectric, and a peak voltage may vary depending on a dielectric constant and thickness of a dielectric layer. Using this, in a normal operating state without ESD, resistance between the ground electrode and the lead wire has an infinite size by this interval, so that current are not conducted therebetween, and a noise filtering function is exhibited.
Unlike this, when a surge current flows due to generation of ESD, ESD may cause an electron tunneling phenomenon so that a space between the ground electrode and the lead wire may be changed so as to have conductivity. Accordingly, ESD flows to the ground electrode in which a current flow path is relatively short compared to a coil electrode, thereby protecting the coil electrode from the ESD.
However, in this structure, the dielectric layer itself serving as ESD protection function may be destroyed depending on a size of the surge current. In addition, even if the surge current flow to the ground electrode, a portion shared in a movement path of current may exist, so that the risk of occurrence of malfunction may be always inherent, whereby the surge current may flow to the coil electrode. Consequently, this may cause a problem such as a reduction in reliability in a product.